The present invention relates to a planar heterojunction avalanche photodiode having a guard ring.
The development of an avalanche photodiode (APD) using an In.sub.0.53 Ga.sub.0.47 As semiconductor is under way for optical communications in the 1 to 1.6 micron wavelength region, where the transmission loss of optical fibers is relatively small. Such a heterojunction APD uses a heterojunction of In.sub.0.53 Ga.sub.0.47 As and InP, which permit lattice matching to each other. The InGaAs layer, whose bandgap is smaller, is used for light absorption, and either the electron or the positive hole carriers, generated therein by optical excitation, are transported to the p-n junction in the InP for avalanche multiplication. For this kind of APD, usually a planar structure is used, wherein a p.sup.+ region is selectively diffused in n-InP to form a stepwise p-n junction. However, there is the problem that, when a reverse bias is applied to the p-n junction, the electric field will most readily concentrate in the circumferential part of the diffused region, resulting in an earlier breakdown than in the p-n junction of the flat part of the diffused region, which is closer to the light receiving area. This phenomenon is known as edge breakdown. To prevent this edge breakdown there is proposed an arrangement in which the circumference of the diffused region is surrounded by a guard-ring of a linearly graded p-n junction, whose breakdown voltage is relatively high.
T. Shirai et al. attempted to prevent edge breakdown by so forming a guard-ring that its linearly graded p-n junction front reaches the same depth as a stepwise p-n junction (T. Shirai et al., "InGaAs Avalanche Photodiodes for 1 .mu.m Wavelength Region", ELECTRONICS LETTERS, 7th July 983, Vol. 19, No. 14, pp. 534-535). However, while the depletion layer in a stepwise p-n junction, upon application of a reverse bias, grows unidirectionally into an n-conductivity region, the depletion layer in a linearly graded p-n junction grows into both p- and n-conductive regions. As a consequence, the problem arises that a contour having a positive curvature emerges in the depletion layer resulting from the two p-n junctions, and an electric field concentrates on the positive curvature region there inviting edge breakdown.
T. Torikai et al. tried to prevent edge breakdown by forming the linearly graded p-n junction front of a guard ring in an even deeper position than the stepwise p-n junction (T. Torikai et al., "Low Noise and High Speed InP/InGaAsP/InGaAs Avalanche Photodiodes with Planar Structure Grown by Vapor Phase Epitaxy", 10th European Conference on Optical Communication, Stuttgart, Sept. 3-6, 1984, Conference Proceedings, pp. 220-221). However, since the guard ring of the linearly graded p-n junction in this structure is positioned close to the InGaAsP layer whose bandgap is smaller, it is susceptible to edge breakdown from the depletion layer's edge of the guard-ring's p-n junction, so that the front position of the linearly graded p-n junction requires critical control.
In the U.S. Pat. application Ser. No. 713,669 or EPC Application No. 85103299.5 jointly filed with the present inventor, there is proposed an APD in which edge breakdown is prevented by providing two linearly graded p-n junctions and easing the curvature of the p-n junction depletion layer jointly formed by a stepwise p-n junction and the two linearly graded p-n junctions. This structure, however, requires a process to produce two guard-rings. Moreover, since it demands a highly precise exposure alignment of the outer guard-ring to the inner one, there are problems of difficult processing and lower element yield.
Furthermore, in every one of the aforementioned prior art examples, the stepwise p.sup.+ -n junction front is arranged in a high-concentration n-InP layer or at the interface between low-concentration n-InP and high-concentration n-InP layers, so that noise reduction is restricted.